Stab Posted June 7, 2014 Share Posted June 7, 2014 Today I noticed that the brightness increases when I zoom in on the Sigma 18-35mm f1.8. When I get to around 28mm I suddenly get like a stop increase in ilumination. I thought this was a constant aperture lens. This means that brightness shouldn't change when zooming, right? My buddy's Panny 12-35mm doesn't do this. Could someone please check / reply if your Sigma does this also? Or is it a defect or does it have something to do with the Speed Booster which I'm using with it? Thanks! ---------------------------------- Edit: I made a video which shows the problem: You can see the problem clearly at F4. At F1.8 it is almost non existent... https://vimeo.com/97710680 (at f4) https://vimeo.com/97710654 (at f1.8) Quote Link to comment Share on other sites More sharing options...
Henry Gentles Posted June 8, 2014 Share Posted June 8, 2014 Did you have the hood on? Quote Link to comment Share on other sites More sharing options...
AKH Posted June 8, 2014 Share Posted June 8, 2014 The Sigma doesn't have completely constant transmission across it's focal length range. http://www.dxomark.com/Lenses/Sigma/Sigma-18-35mm-F18-DC-HSM-A-Canon-mounted-on-Canon-EOS-700D---Measurements__870 Would the speedbooster make it more noticeable? Not sure. cpc 1 Quote Link to comment Share on other sites More sharing options...
Stab Posted June 8, 2014 Author Share Posted June 8, 2014 Did you have the hood on? Nope. Quote Link to comment Share on other sites More sharing options...
Stab Posted June 8, 2014 Author Share Posted June 8, 2014 I just tested some more. The difference between brightness at different focal lengths is actually less visible wide open and becomes more apparent when stopping down a bit. The lens is most bright at 28mm with a pretty large difference with 20-24mm and gets slightly less bright at 35mm again. Someone with an Sigma 18-35mm who can look how theirs is doing? Quote Link to comment Share on other sites More sharing options...
jgharding Posted June 8, 2014 Share Posted June 8, 2014 Hmmm this lens is interesting. Since it's APSC only, and f1.8 x1.6 is f2.8 (roughly) I'd guess it's actually a full frame design for a 28mm to 55mm f2.8, or therabouts, with a "speedboosting" element at the back. I think my maths is funk there :/ but still, it'd make sense if this lens incorporated a focal reducer. That would explain why it's faster and sharper than most expect. So effectively you're going through two speedboosters! Maybe this accentuates transmission inconsistencies? Just a guess though Quote Link to comment Share on other sites More sharing options...
Stab Posted June 8, 2014 Author Share Posted June 8, 2014 Hmmm this lens is interesting. Since it's APSC only, and f1.8 x1.6 is f2.8 (roughly) I'd guess it's actually a full frame design for a 28mm to 55mm f2.8, or therabouts, with a "speedboosting" element at the back. I think my maths is funk there :/ but still, it'd make sense if this lens incorporated a focal reducer. That would explain why it's faster and sharper than most expect. So effectively you're going through two speedboosters! Maybe this accentuates transmission inconsistencies? Just a guess though Hmm sounds plausible. I just saw a DXO transmission test of this lens, and it confirms the brightness increase at 28mm but it is only very very slightly. On my lens it is much more apparent it is at least a stop in increase. Maybe this is indeed because it is 'Speed Boosted' twice. Would this have any other disadvantages? Or can you, in theory, keep Speed Boosting lenses forever with multiple boosters? :D Quote Link to comment Share on other sites More sharing options...
jgharding Posted June 8, 2014 Share Posted June 8, 2014 Hahah I like the idea of endless boosters on a tiny sensor!! I have no idea to be honest... I'd guess you can leep fpcussing onto tinier and tinier points, much like when you start a fire using a magnifying glass. It's always just counteracting the light loss from a smaller sensor area, so if it's worthwhile or not I can't say. They only need to exist because smaller sensors exist. But I don't see why you couldn't just keep boosting down if they're all high quality. Quote Link to comment Share on other sites More sharing options...
Stab Posted June 8, 2014 Author Share Posted June 8, 2014 Anyway. I'm sure there are some users here with the Sigma 18-35... Anybody can check theirs? Quote Link to comment Share on other sites More sharing options...
Julian Posted June 8, 2014 Share Posted June 8, 2014 Can you make a video showing the issue? Definitely doesn't sound normal. There will be a difference in transmission and vignetting might be different for each focal length so there will be difference in corner brightness. But the way yoj describe it sounds different... Quote Link to comment Share on other sites More sharing options...
Axel Posted June 9, 2014 Share Posted June 9, 2014 I made some zoom shots with the Sigma. What I found confirmed is that it is not really parfocal (focus changes slightly, at least or most noticeable with open aperture). There definitely was no change in brightness. However, although the zoom is rather smooth for a manual zoom ring, the 2x zoom range imo doesn't justify to use it as a transition (to point to a detail or to reveal it's surroundings). Quote Link to comment Share on other sites More sharing options...
Stab Posted June 9, 2014 Author Share Posted June 9, 2014 Ok, I made a test video. You can see the problem clearly at F4. At F1.8 it is almost non existent... https://vimeo.com/97710680 (at f4) https://vimeo.com/97710654 (at f1.8) Quote Link to comment Share on other sites More sharing options...
AaronChicago Posted June 9, 2014 Share Posted June 9, 2014 Theres definitely something wrong. Mine doesn't do that. What camera are you using, and is everything set to manual? Quote Link to comment Share on other sites More sharing options...
Stab Posted June 9, 2014 Author Share Posted June 9, 2014 Theres definitely something wrong. Mine doesn't do that. What camera are you using, and is everything set to manual? Yes everything at manual. I'm using a Panny GH3 and a Speed Booster. Image is really good though. Just wondering if something is wrong and if I'm getting too few light at certain aperture + focal lenght combinations. I found someone over at the Dpreview forum who tested after me, and found similar results... Quote Link to comment Share on other sites More sharing options...
AaronChicago Posted June 9, 2014 Share Posted June 9, 2014 Is it a metabones speedbooster or a chinese focal reducer? Quote Link to comment Share on other sites More sharing options...
Administrators Andrew Reid Posted June 9, 2014 Administrators Share Posted June 9, 2014 Hey Stab I know what is going on here... Had it on mine. The mount is slightly loose to the Nikon Speed Booster so as you twist the lens barrel to zoom, it moves in the mount very fractionally and the aperture lever gets shifted around with it. Try tightening it by levering out the mounting things* on the adapter. * I have no idea what the name is for these!! Julian and dahlfors 2 Quote Link to comment Share on other sites More sharing options...
Stab Posted June 9, 2014 Author Share Posted June 9, 2014 Hey Stab I know what is going on here... Had it on mine. The mount is slightly loose to the Nikon Speed Booster so as you twist the lens barrel to zoom, it moves in the mount very fractionally and the aperture lever gets shifted around with it. Try tightening it by levering out the mounting things* on the adapter. * I have no idea what the name is for these!! Thanks Andrew. I'm not exactly sure what you mean. But first, are we talking about the same problem? Indeed, the lens and the Speed Booster are not a 100% fit. When I put a bit pressure on the lens and twist it, it 'clicks' and there is an increase in brightness. However, this is not related to this problem I think. Even when I zoom, while pushing the lens in the opposite direction, this problem occurs. It might be related, but I don't think so. I just tied the screws a bit firmer. I mean those screws on the Speed Boosters metal mount. Do you mean those? Or did you mean something else? Thanks for helping. Really appreciated. Quote Link to comment Share on other sites More sharing options...
Stab Posted June 9, 2014 Author Share Posted June 9, 2014 I really start to think that this is just how this lens is build. Although I found 1-2 people reporting the same 'problem', I've heard more people say that theirs doesn't do this. I just pointed the lens at me, and looked at the lens opening while turning the zoom ring. When zooming in, the 'hole' naturally becomes larger. A longer focal lenght, needs a bigger aperture opening so that's normal. But when I get to arround 28mm, the opening seems to 'exponetially' become larger. Because not only does the 'inner ring' shift, but at that focal length the outer aperture also opens further. This is visible at all aperture settings, but moreso at F3.5 and onwards. I'm pretty sure this is what is causing the extra light to come in starting at +- 28mm. Now the only question that remains is... Is this normal for this lens? Or is it a 'defect' in mine and I do I receive too few light at focal lenghts under 28mm.... Quote Link to comment Share on other sites More sharing options...
MarcoB Posted August 4, 2015 Share Posted August 4, 2015 I really start to think that this is just how this lens is build. Although I found 1-2 people reporting the same 'problem', I've heard more people say that theirs doesn't do this. I just pointed the lens at me, and looked at the lens opening while turning the zoom ring. When zooming in, the 'hole' naturally becomes larger. A longer focal lenght, needs a bigger aperture opening so that's normal.But when I get to arround 28mm, the opening seems to 'exponetially' become larger. Because not only does the 'inner ring' shift, but at that focal length the outer aperture also opens further. This is visible at all aperture settings, but moreso at F3.5 and onwards. I'm pretty sure this is what is causing the extra light to come in starting at +- 28mm. Now the only question that remains is... Is this normal for this lens? Or is it a 'defect' in mine and I do I receive too few light at focal lenghts under 28mm....Hi Stab, I have the same problem. I did some research and seems that we are not alone. Sigma 18-35 has a problem, and it is not a little one, it has not a constant aperture if you record a complete zoom in (from 18-35 ) or zoom out you will have a luminance change. Andrew should add this information to his useful Gh4 guide. I bought sigma 18-35 and nobody told me about this problem: not guides, posts, reviews, video reviews. Quote Link to comment Share on other sites More sharing options...
jonpais Posted August 4, 2015 Share Posted August 4, 2015 I've been testing the GH4 with the Sigma and Speed Booster XL 0.64 for the past couple of weeks and haven't experienced any noticeable change in brightness like you describe. And just to be sure, I just took mine out and did a couple of zooms and see no discernible difference in luminosity. Of course, there will be minor changes in luminosity with any zoom, even those with constant aperture. But to assert that the lens is defective, or to demand that a reviewer is obligated to mention the obvious, is going a bit too far, IMHO. Quote Link to comment Share on other sites More sharing options...
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